Microplastics are found in places people rarely imagine. They permeate the deepest oceans, reach the highest mountains, drift in the air, and even lie in wait at the world’s most remote poles. Recent studies reveal that these contaminants reach spots untouched by humans for decades. In two new papers, researchers from Saint Louis University in the United States report high concentrations of microplastics in Cliff Cave, a system in Missouri that has been closed to visitors for 30 years.
Elizabeth Hasenmueller, deputy director of the university’s environmental institute, and her team published their findings in journals focused on environmental science and water research. They detected notable levels of microplastics throughout Cliff Cave in Saint Louis County, Missouri.
Microplastics are plastic particles smaller than 5 millimeters and appear in marine, terrestrial, and freshwater environments. Hasenmueller previously explored microplastics in river basins like the Meramec River but shifted attention to subterranean environments, an area that has received relatively limited study.
Research on microplastics began with oceans due to the visible pollution there, but interest has grown in rivers, lakes, and other surface waters. The evidence now points to significant presence beneath the surface as well.
One of the least explored areas in this field is the subsurface. These particles can reach groundwater, a common drinking water source, or caves where delicate ecosystems depend on clean conditions.
Closed to the public since 1993
The Cliff Cave study site was chosen because it has been off-limits to the public since 1993. This restriction helps ensure that human presence did not skew contamination results. The research found microplastics throughout the cave, with the highest concentrations near the entrance and in the sediments.
The team noted that access controls in the area reduce the chance that fibers shed by clothing or litter from visitors would explain the findings. Flooding events were also considered, as moving water can transport microplastics into the cave system.
Floodwaters were observed to drive microplastics into Cliff Cave, often wrapping debris such as plastic bags with leaves and other surface material. The sediments held far more microplastics than the water itself—nearly 100 times more—indicating long-term deposition as water moved through the cave and then receded. Microplastics deposited in sediments tended to stay there for years or even decades, while waterborne concentrations dropped as water levels fell.
When floods recede, microplastics tend to accumulate around the cave mouth rather than deep inside. The researchers found that the cave’s main entrance hosts a large pool of microplastic waste, likely caused by flood deposition or the settling of airborne particles near the opening.
Floods bring microplastics in
The researchers observed that floods carry microplastics into the cave, often catching debris such as leaves and acorns in plastic fibers. The flood-driven transport, along with sedimentation, explains why sediments harbor most of the microplastics. The study found that almost all detected microplastics—about 99 percent—were stored in cave sediments, with only a small fraction present in the water at any given time.
As water levels rise during floods, the variety and abundance of microplastics in the water increase. After flooding, the settled particles remain in the sediments, persisting for decades. When a dry period returns, the water’s microplastic concentration drops significantly.
Near a residential area
Even though the cave is isolated from direct human contact, it is still influenced by nearby populations. Cliff Cave lies close to residential zones, which may contribute microplastics through various pathways. This aligns with earlier findings showing that population density strongly influences where microplastics are found in the environment. With these insights, there are practical steps people can take to reduce their impact.
Reducing personal plastic use remains challenging, but awareness helps. Choosing sustainable textiles and limiting unnecessary plastic consumption can lower the amount of microplastic fibers entering the environment. Broadly, reducing overall plastic production and consumption supports the protection of fragile underground habitats from emerging contaminants.
Limited studies have evaluated microplastics in underground ecosystems. This research provides resource managers with data to safeguard these sensitive habitats from microplastics and related contaminants. The references support the findings, including studies published in scientific journals on total environmental science and water research, with noted DOIs for further reading: 10.1016/j.scitotenv.2023.164690 and 10.1016/j.watres.2023.120204.
Further exploration of subterranean microplastics will help clarify how much of the material is actively moving through cave systems versus how much is stored long-term in sediments. A key insight is that sediments act as a long-term sink for microplastics, while water carries a temporary load during flood events. As environmental conditions shift, the balance between sediment storage and waterborne microplastics can guide conservation and water quality strategies.
This research emphasizes the role of population dynamics and human activity in shaping microplastic distributions. It reinforces the idea that individual choices matter and that collective action can reduce the spread of microplastics into vulnerable underground environments.
References and further reading are provided through the formal DOI citations above to support the study’s findings and aid researchers in related fields.